Transcript Elements, Compounds and Molecules

Elements, Compounds
and Molecules
Section 5.3
• In total, 117 elements have been observed
as of 2008, of which 92 occur naturally on
Earth – the others are man-made!
• Every kind of matter has to be made up of
one or more of these 117 elements.
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• Matter that is made up of only one type of atom
is called an element (like those on the periodic
table)
• The atoms of any one element are exactly alike
but the atoms of any given element are different
from atoms of every other element:
– Example:
• All hydrogen atoms are the same as all other hydrogen
atoms but they are not the same as oxygen atoms
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• Most of the substances we know are made
up of two or more elements:
– Water (H2O), salt (NaCl), carbon dioxide,
(CO2) and baking soda (NaHCO3)
• These substances are possible because
atoms of different elements can link-up
• Elements are pure substances that cannot
be broken down into simpler parts by
ordinary chemical means
• Elements combine to produce compounds
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• Some common elements are gold (Au),
copper (Cu), hydrogen (H), oxygen (O)
• Most elements are found in a solid state
but they can also be gasses and liquids
(there are only 2 liquids)
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• A compound is a substance made up of two or
more elements
• When elements combine to make compounds
they lose their own properties; the new
compound has it’s own properties
• Compounds don’t have to be in the same state
as the elements from which they are made
• Example:
– hydrogen (gas) and oxygen (gas) make water (liquid)
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Molecules
• A molecule is the smallest part of a
compound that still has the properties of
the compound
• It must have at least two atoms but can
have thousands.
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Atomic Theory
Explaining Chemical Facts and Laws
Dalton’s Atomic Theory
• All matter is made up of small particles called
atoms
• Atoms cannot be created, destroyed or divided
into smaller particles
• All atoms of the same element are identical in
mass and size, but they are different in mass
and size from the atoms of other elements
• Compounds are created when atoms of different
elements link together in definite proportions
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• Dalton’s model uses the idea that
elements are different because their
“particles” are different
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Laws vs. Theories
• In science:
– Laws do not explain anything, they just
describe and summarize
– Theories explain why something happens
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Atomic Masses
• Atomic mass is the mass of an average
atom of an element
• Hydrogen was the lightest known element
at the time so Dalton gave it an atomic
mass of 1
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• Since the atomic mass of Hydrogen was 1
Dalton theorized that all other atomic
masses could be compared to the mass of
Hydrogen.
• For example, the mass of Lithium is 3 so it
is 3 times larger than Hydrogen etc.
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• In Dalton’s New System of Chemical Philosophy,
published in 1808, Dalton provided relative
masses for 20 elements to support his atomic
theory.
• He also showed 17 examples of how the theory
explained the formation of compounds by
proving that H2O has an atomic mass of 10
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